Six infants with acute megakaryoblastic leukemia and a translocation (1;22)(p13;q13) were studied. There were five female infants and one male infant, and the age at initial examination varied from 0.8 to 6.5 months (median, 2.3 months). All the patients had hepatosplenomegaly and anemia (6 to 8.3 g/dL), and four patients had thrombocytopenia (9,000 to 63,000/mm3). The bone marrow showed prominent fibrosis in five cases and reticulin fibrosis in one patient at presentation. Crush artifact often made the histologic sections difficult to interpret, but typical megakaryoblasts could be identified in the smears. Biopsy specimens of the liver and lymph node were suggestive of a nonhematopoietic malignant condition because of the cohesiveness of the tumor cells, stromal fibrosis, and the prominent sinusoidal and vascular pattern of infiltration. Immunophenotyping of peripheral blood mononuclear cells was helpful in identifying the blasts as belonging to the megakaryoblastic lineage. Using a panel of mononclonal antibodies, it was also possible to confirm the nature of the infiltration in paraffin sections and to differentiate it from other childhood small round cell tumors, especially neuroblastoma in paraffin sections (typical staining pattern: CD45-, CD43+, vW Factor, Ulex europeus I+, CD20-, CD45RO-, synaptophysin-, chromogranin-, cytokeratin-, desmin-). This special type of infantile acute leukemia can be recognized with confidence if one is aware of its clinical features, peculiar pathologic characteristics, the morphologic features and immunophenotype of the megakaryoblasts, and the unique cytogenetic abnormality.